Dynamic membranes are used in the purification of liquids, especially in the demineralization of water. Such dynamic membranes consist essentially of two components. The first component is a microporous, continuous structure such as ceramic material, sintered metal material and the like. If liquids are pressed through such a microporous, continuous structure, scarcely any impurity is held back.
It has therefore previously been proposed to cover said microporous structure with a second component comprising a thin layer of discrete particles. These particles have dimensions which in all directions are of the same order of magnitude.
The combination of the microporous structure and the layer of particles is called a "dynamic membrane".
If a liquid flows through such a dynamic membrane, part of the impurities are held back, but some of them traverse the membrane. If there are 100 parts of impurities present in the original liquid and 70 parts are retained and only 30 parts traverse the membrane, it is said that the retention factor, R, is 70%.
A higher value for R indicates a better removal of impurities, i.e., less impurities, such as minerals, are found in the effluent that has passed through the dynamic membrane, e.g., water.
A high value for R, however, necessitates a very microporous structure and a very even distribution of the discrete particles which cover the microporous structure. Such a system results in a dynamic membrane that has a high flow resistance. Thus, in order to obtain a reasonable amount of the effluent, the pressure drop across the membrane should be high.
The amount of purified liquid is expressed by F = flux, which is measured in m.sup.3 /m.sup.2 /sec, i.e., the volume of effluent per area of dynamic membrane per second.
Although in principle a dynamic membrane can be used in removing impurities from any liquid, a well-known use is the demineralization of water, and more specifically, the desalination of water. The general use of a dynamic membrane is therefore called "reverse osmosis".
A high retention factor, R, indicates that the effluent, e.g., water, is devoided of minerals as much as possible.
An attractive feature of reverse osmosis with a dynamic membrane is that if, for one cause or another, the dynamic membrane becomes "blocked" (i.e., the flux, F, becomes low), the direction of flow of the liquid from the effluent side to the original side may be reversed for a short time. During such a reversal of flow, the particles are blown away from the microporous structure and whirl in the original liquid. If necessary, fresh particles can be added to the original liquid. If the flow of the liquid is again reversed to its original direction, the particles settle down on the microporous structure, and the dynamic membrane will again function as usual.
A convenient way to "build up" a dynamic membrane is to have one side of the microporous structure in contact with the liquid to be purified. Then the discrete particles are dispersed in this liquid or a dispersion of the particles is added to the liquid. Finally, pressure is applied to the liquid. The liquid starts to flow through the porous structure, moving the particles into the openings of the microporous structure and forming the dynamic membrane.
The nature of the microporous structure is not important provided the structure is microporous. Of course, the microporous structure should not dissolve in the liquid to be filtered. The microporous structure may consist of an inert material, or it may be made of an ion exchanging material.
The discrete particles should also be insoluble in the liquid to be filtered. The form of discrete particles used in the prior art is not optimally suited to the formation of dynamic membranes. The particles should seal the micropores, but they do not necessarily have to have dimensions of the same order of magnitude in all directions. Moreover, discrete particles are rigid, and therefore, they do not optimally seal the micropores.
It is therefore preferred that discrete particles are replaced by thin foils. Such foils have a structure so that one dimension of the foil is much smaller than the other two dimensions. However, the two larger dimensions should be such that the micropores are sealed. In other words, in the direction perpendicular to the two larger dimensions the foils are thin.
During reverse osmosis, a dynamic membrane comprised of thin foils offers much less resistance to the flow of the liquid so that normal flux is attained at a lower pressure. In other words, if pressures are the same as usual the flux of the dynamic membrane of the present invention is higher than that of a dynamic membrane with the discrete particles which are more or less spherical.
The dynamic membranes of the present invention have a quite acceptable retention factor.